My company currently employ contrators who reside in a lock down area in North of England .

During weekdays , Contractors (who reside in lock down area ) live in shared accomadation . Is this permitted   

Given the different location specific rules you may wish to provide a little more information as even in the same northern town the rules can change dependent upon which (election) ward they are in.

That said during the week they would be considered to be a single household - you are right though to question if they return "home" over the weekend as this is rupturing the bubble and allowing a route for infection entry.

Is it permitted? that depends upon whose rules you are discussing - government, devolved assembly, local county or district council.....

Want to control it? you have as much chance of identifying a foreign holiday destination you can travel to and not be exposed to potential quarantine upon your return as identifying an area contractors can be drawn from.